The therapeutic drug level refers to the appropriate level of a medication in the blood, enough to produce the desired effect but not enough to cause harm (toxicity). For some medications, this therapeutic level is a relatively narrow range, and for these, it is helpful to measure the level periodically to avoid under- or overdosing
Examples of medications that need monitoring include certain antibiotics, such as gentamicin and vancomycin, seizure medications, such as phenytoin (i.e. Dilantin), and heart medicines, such as digoxin (i.e. Lanoxin).
To measure the amount of a medication in the bloodstream, a blood sample is needed. Blood is usually taken from a vein on the forearm. First, the skin over the vein is cleaned with an antiseptic. Next, a strong rubber tube, or "tourniquet," is wrapped around the upper arm. This enlarges the veins in the lower arm by restricting blood flow through them.
A fine needle is gently inserted into a vein, and the tourniquet is removed. Blood flows from the vein through the needle and is collected in a syringe or vial. The blood is then sent to the laboratory for testing. After the needle is withdrawn, the puncture site is covered for a short time to prevent bleeding.
A person should request specific instructions from his or her healthcare professional. Generally, no preparation is required.
Each of the medications that needs to be monitored has a specific level that the healthcare professional aims for. If the level is too high, the provider will reduce the amount of medication taken or increase the time between doses. If the level is too low, the provider will increase the amount of medication taken or decrease the time between doses.